First off I wish to say that humans have original sin and that all humans have it. I see it more as a spiritual issue rather then a biological one that cannot be solved by science. It starts in the heart at birth and over time with maturity it grows in us and we become evil in our thoughts in ways such as not listening to our parents when we are young, stealing toys, being means and as we get older it gets much worse.
Humans did have knowledge of what was good and evil and we got it from God. The issue was that when we ate of the Forbidden Fruit, we decided on what was evil and good outside of God and thus became like our own gods defining what is right and wrong. God gave the first humans divine inspiration to have knowledge and the capacity to do so. The issue is not that we had now knowledge, but the issue is that we decided to no longer trust what God had told us what was right and wrong and decided to define it ourselves.